The present invention relates to a pattern formation method for use in fabrication process and the like for semiconductor devices.
In accordance with the increased degree of integration of semiconductor integrated circuits and downsizing of semiconductor devices, there are increasing demands for further rapid development of lithography technique. Currently, pattern formation is carried out through photolithography using exposing light of a mercury lamp, KrF excimer laser, ArF excimer laser or the like, and use of F2 laser lasing at a shorter wavelength is being examined. However, since there remain a large number of problems in exposure systems and resist materials, photolithography using exposing light of a shorter wavelength has not been put to practical use.
In these circumstances, immersion lithography has been recently proposed for realizing further refinement of patterns by using conventional exposing light (M. Switkes and M. Rothschild, “Immersion lithography at 157 nm”, J. Vac. Sci. Technol., B19, 2353 (2001)).
In the immersion lithography, a region in an exposure system sandwiched between a projection lens and a resist film formed on a wafer is filled with a solution having a refractive index n, and therefore, the NA (numerical aperture) of the exposure system has a value n·NA. As a result, the resolution of the resist film can be improved.
Now, a first conventional pattern formation method using the immersion lithography will be described with reference to FIGS. 8A through 8D.
First, a positive chemically amplified resist material having the following composition is prepared:
Base polymer: poly((norbornene-5-methylene-  2 gt-butylcarboxylate) - (maleic anhydride)) (whereinnorbornene-5-methylene-t-butylcarboxylate:maleicanhydride = 50 mol %:50 mol %)Acid generator: triphenylsulfonium nonaflate0.05 gSolvent: propylene glycol monomethyl ether acetate  20 g
Next, as shown in FIG. 8A, the aforementioned chemically amplified resist material is applied on a substrate 1 so as to form a resist film 2 with a thickness of 0.35 μm.
Then, as shown in FIG. 8B, while supplying water 3A onto the resist film 2, pattern exposure is carried out by irradiating the resist film 2 with exposing light 4 of ArF excimer laser with NA of 0.65 through a mask 5. Although a projection lens for condensing the exposing light 4 having passed through the mask 5 on the surface of the resist film 2 is not shown in FIG. 8B, a region sandwiched between the projection lens and the resist film 2 is filled with the water 3A. Thus, an exposed portion 2a of the resist film 2 becomes soluble in an alkaline developer because an acid is generated from the acid generator therein while an unexposed portion 2b of the resist film 2 remains insoluble in an alkaline developer because no acid is generated from the acid generator therein.
After the pattern exposure, as shown in FIG. 8C, the resist film 2 is baked with a hot plate at a temperature of 110° C. for 60 seconds, and the resultant resist film is developed with a 2.38 wt % tetramethylammonium hydroxide developer (alkaline developer). In this manner, a resist pattern 6A made of the unexposed portion 2b of the resist film 2 can be obtained as shown in FIG. 8D.
Next, a second conventional pattern formation method using the immersion lithography will be described with reference to FIGS. 9A through 9D.
First, a positive chemically amplified resist material having the following composition is prepared:
Base polymer: poly((norbornene-5-methylene-  2 gt-butylcarboxylate) - (maleic anhydride)) (whereinnorbornene-5-methylene-t-butylcarboxylate:maleicanhydride = 50 mol %:50 mol %)Acid generator: triphenylsulfonium nonaflate0.05 gSolvent: propylene glycol monomethyl ether acetate  20 g
Next, as shown in FIG. 9A, the aforementioned chemically amplified resist material is applied on a substrate 1 so as to form a resist film 2 with a thickness of 0.20 μm.
Then, as shown in FIG. 9B, while supplying perfluoropolyether 3B onto the resist film 2, pattern exposure is carried out by irradiating the resist film 2 with exposing light 4 of F2 laser with NA of 0.60 through a mask 5. Although a projection lens for condensing the exposing light 4 having passed through the mask 5 on the surface of the resist film 2 is not shown in FIG. 9B, a region sandwiched between the projection lens and the resist film 2 is filled with the perfluoropolyether 3B. Thus, an exposed portion 2a of the resist film 2 becomes soluble in an alkaline developer because an acid is generated from the acid generator therein while an unexposed portion 2b of the resist film 2 remains insoluble in an alkaline developer because no acid is generated from the acid generator therein.
After the pattern exposure, as shown in FIG. 9C, the resist film 2 is baked with a hot plate at a temperature of 100° C. for 60 seconds, and the resultant resist film is developed with a 2.38 wt % tetramethylammonium hydroxide developer (alkaline developer). In this manner, a resist pattern 6B made of the unexposed portion 2b of the resist film 2 can be obtained as shown in FIG. 9D.
As shown in FIGS. 8D and 9D, however, each of the resist patterns 6A and 6B formed by the first and second conventional pattern formation methods is in a defective T-top shape.
Since the positive chemically amplified resist material is used in each of the first and second conventional pattern formation methods, the resist pattern 6A or 6B is in the T-top shape. When a negative chemically amplified resist material is used instead, the resultant resist pattern is in a defective shape with round shoulders.
When a resist pattern in such a defective shape is used for etching a target film, the resultant pattern is also in a defective shape, which disadvantageously lowers the productivity and the yield in the fabrication process for semiconductor devices.